Hi Guys,
I was working on expansion of X450A extreme switches recently in a network where one of the witch was running live and catering to the traffic. With help from some posts and research, here is the procedure with which you can acheive this stacking activity:
1). Check that both master and expansion switch should be running on the same "Image"
Image Selected:   primary                      You can check this with the help of "show switch" command.
2). Unconfigure the expansion switch so that its fresh as new.
3). for "enable stacking", mark the answer as "no", since we'll be using the manual method.
4). Connect the stacking cable form master switch to expansion switch.
5). Run "show stacking" on master switch and you will see the MAC of expansion switch also now.
Node  MAC Address    Slot  Stack State   Role     Flags
    ------------------  ----   -----------  -------  ---
     00:04:96:6d:3f:6a  -      Disabled     Master   ---
    *00:04:96:83:4e:b1  1      Active       Master   CA-
5). Run the following commands on the master switch:
configure stacking  node-address  slot-number 2
configure stacking  node-address  license-level  advanced-edge
configure  stacking node-address  master-capability off
synchronize  stacking node-address 
reboot node-address 
6). Now once your expansion switch comes up after the reboot, It will be stacked with the master switch. And this procedure does not hamper any traffic going through the master switch.
Node  MAC Address    Slot  Stack State   Role     Flags
  ------------------  ----  -----------  -------  ---
  00:04:96:6d:3f:7c   2     Active        Standby   CA-
  *00:04:96:7e:04:ed  1      Active       Master   CA-
  
you can verify your stack using "show switch" and "show slot" on the master switch.
PS: I hope this is helpful for you.
Regards
Pardeep Singh